Comments on: important warning to postfix users

By: anonymous

anonymous — Sun, 25 Nov 2007 12:30:03 +0000

I know people are getting lazy, but please please can you use capital letters at the beginning of your sentences. It really isn’t much work and it makes reading much easier for lots of us.

Thanks :-)

By: desrt

desrt — Sun, 25 Nov 2007 09:55:59 +0000

it is actually the default that relay_domains includes mydestination. i think probably the server won’t accept anything not in relay_domains (so really it’s actually more like “accept_mail_for_domains”)

By: Peter Nelson

Peter Nelson — Sun, 25 Nov 2007 08:59:57 +0000

Basically you shouldn’t add domains in mydestination to relay_domains. If your server is the destination, then it is not relaying them.

Anyway, the biggest problem with back scatter is, of course, with older MS Exchange servers where you can’t turn off its bounces.

By: Oded

Oded — Sun, 25 Nov 2007 01:21:51 +0000

When acting as a relay for trusted clients then you need to be able to bounce messages you accepted for delivery but couldn’t actually deliver – and this is the situation with most mail servers: normally clients don’t deliver directly to their destination but through a local MTA that trust them. This is a good thing, because it allows MTA operators to blacklist all IP addresses that aren’t expected to be used by other MTAs (such as dialups) and thereby minimize the impact of SPAM zombies.

Also, some servers relay incoming mail for other hosts (more common scenario then what you would expect – for example in DMZs), in which case bounce messages are also expected, though in such cases the administrator would do well to make sure the front-end MTA can resolve and verify recipient addresses during RCPT instead of during delivery.

By: desrt

desrt — Sun, 25 Nov 2007 00:15:45 +0000

there’s one legitimate case for sending bounces: when you are acting as a relay for trusted clients.

By: Anonymous

Anonymous — Sat, 24 Nov 2007 23:53:14 +0000

This seems like *two* problems, actually: first, that postfix interprets a setting in a way that will cause it to accept mail it can’t deliver, and second, that postfix generates bounces under any circumstances.

Seriously, should an MTA *ever* generate a bounce? It should reject a mail or deliver a mail, but never accept and then bounce a mail under any circumstances.

By: Paul Collins

Paul Collins — Sat, 24 Nov 2007 23:26:10 +0000

I just checked my postfix configuration, and it turns out that having reject_unknown_recipient_domain in smtpd_recipient_restrictions also prevents postfix from accepting mail for random subdomains. This may work out better for folks with a lot of subdomains to administer.